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  <h1 class="title">简介</h1>
  <p class="para">
   In order to use the XML Data Access Service for Service Data Objects,
   you will need to understand some of the concepts behind SDO:
   the data graph, the data object, XPath and property expressions,
   and so on.
   If you are not familiar with these ideas, you might want to look first at
   <a href="ref.sdo.html" class="link">the section on SDO</a>.
  </p>
  <p class="para">
   The job of the XML DAS is to move data between the
   application and an XML data source, which can be either a file or
   a URL. SDOs are always created and maintained according to a model 
   which defines type names and what property names each type may have.
   For data which is from XML, this SDO model is built from a schema file 
   written in XML schema language (an xsd file). This schema file is 
   usually passed to the create method when the XMLDAS is initialised. 
   The 
   <a href="http://www.ibm.com/developerworks/webservices/library/specification/ws-sdo/" class="link external">&raquo;&nbsp;SDO 2.0 specification</a>
   defines the mapping between XML types and SDO types.
   There are a number of small limitations in the PHP support -
   not everything which is in the specification can be done - and
   these limitations are summarised in a later section.
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